Field of the Invention
Embodiments of the present invention generally relate to power converters and, more particularly, to a method and apparatus for three-phase AC-AC power conversion.
Description of the Related Art
Polyphase AC-AC converters are extremely common in the industrial sector to drive motors, with the ability to impart a given torque and therefore speed to a motor. Motors are typically induction machines (IM) but can also be permanent magnet machines (PMM), synchronous reluctance machines (SynRM) or synchronous machines (SM). The common objective is to use energy from a three-phase low voltage (LV) or medium voltage (MV) AC bus running at standard grid frequency (e.g. 50, 60 Hz or 400 Hz) and send it to the motor while controlling the motor waveforms to achieve the proper motor operation. The output AC voltage might be lower or larger than the grid voltage and the frequency might be significantly lower or larger than the grid frequency. These products are called variable frequency drives (VFD). Other applications, such as mechanically driven power generators (such as a diesel generator), solid state transformers, frequency changers, and the like, require similar functions.
Conventional power converters and variable frequency drives employing either DC Links, AC-Links with pulsed density modulation signals, or matrix converters have been developed for three-phase applications. Each converter topology finds its own set of challenges, whether harmonic content of either the input or output waveforms, output dV/dt, EMC issues, input voltage sensitivity or restricted voltage transform ratios. Such traditional topologies are voltage source inverters (VSI), back to back VSI (BB-VSI), cycloconverters, and matrix converters.
VSIs represent the most common implementation for the VFD market but are plagued by poor distortion and power factor on the grid side. They can only “buck” (reduce) the voltage on the motor side, and they cannot regenerate braking energy to the grid side. They are the simplest devices and only need six diodes and six main switches to operate, yet they do require a large hold-up capacitor which has a low reliability.
BB-VSIs use two VSIs, one used in a boost AC-DC mode and one used in a buck DC-AC mode. There is a DC link connecting the two VSIs. The grid side distortion is much better than with a VSI, boosting is possible, but the complexity is twice the normal VSI' s.
Few equipment has used cycloconverters. This system is plagued by a very high circulating current in the high-frequency (HF) link due to the discrete pulse nature of the resonant link. This approach is therefore quite inefficient compared to other approaches. Matrix converters have emerged as a product category. Although they are improving some issues, they have the highest number of switches, eighteen.
Therefore, there is a need in the art for improved three-phase AC-AC series resonant conversion.